US3461193A - Novel procedure for starting the flash-extrusion of expandable resin compositions - Google Patents

Description

Aug. 12, 1969 J, GILARm 3,461,193

NOVEL PROCEDURE FOR STARTING THE FLASH-EXTRUSION v OF EXPANDABLE RESIN COMPOSITIONS Filed Jan. 4, 1967 lNV ENT OR fUJL'KTZ/QHN 6/mk04 ATTORNEY United States Patent ice NOVEL PROCEDURE FOR STARTlNG THE FLASH-EXTRUSHON OF EXPANDABLE RESIN COMPOSITIONS Robert John Gilardi, Richmond, Va., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Jan. 4, 1967, Ser. No. 607,304 Int. Cl. 1329f 3/06 U.S. Cl. 26453 2 Claims ABSTRACT OF THE DESCLOSURE In flash extrusion of foams, sponges and plexifilamentary strands a polymer solution at high temperature and pressure is extruded into a region of reduced pressure, resulting in flashing of the solvent and solidification of the polymer. In starting up, inert gas under pressure is supplied to the enclosed path between the solution supply valve and the extrusion orifice to prevent premature flashing of the solvent and solidification of the polymer.

BACKGROUND This invention relates to the flash-extrusion of polymer/liquid mixtures and more particularly to an improved procedure for beginning flow of the mixtures through a die having at least one flow-restricting extrusion orifice.

Flash-extrusion, as herein referred to, is a process wherein a polymer/ liquid dispersion or solution at elevated temperature and pressure passes through a flow-restricting orifice into a region of reduced pressure such that substantially all the liquid is vaporized almost instantaneously and the polymer stream is simultaneously greatly expanded and cooled to solidification by the vaporization of the liquid. Products of so-defined flash-extrusion include closed-cell foams, foams with interconnecting cells, polymer sponges, and fibrillated plexifilamentary strands. These products are generally in the form of filaments, yarns, or thin sheets.

More specifically, flash-extrusion as defined herein deals with the extrusion of an expandable composition comprising a thermoplastic polymer and a liquid, said liquid having an atmospheric boiling temperature well below (i.e., at least C. below) the polymer melting temperature and being a non-solvent for the polymer at or below the said boiling temperature. Before extrusion into a region of sharply reduced pressure, the expandable composition is at a temperature above that at which polymer freezes out and under a pressure at least suflicient to maintain the liquid state. Said liquid is present at between about 98% by weight of the composition and a minimum amount sufiicient to so rapidly precipitate the polymer on adiabatic vaporization that molecular orientation is frozen into the solidfied structure. This minimum amount is ordinarily about by Weight of solution.

A typical and preferred flash-extrusion process is that disclosed by Blades and White in U.S. 3,227,784, the disclosure of which is incorporated herein by reference. The procedure of the present invention, however, is not limited to the process of the aforesaid patent, particularly as regards its use only of crystalline polymers. By way of illustrating further the preferred processes to which the procedure of this invention is directed, reference is made to U.S. Patent No. 3,227,664 dealing with closed-cell ultramicrocellular structures and to U.S. Patents Nos. 3,081,519 and 3,227,794 dealing with fibrillated plexifilamentary strands.

The viscosities of expandable compositions suitable for flash-extrusion are much lower than normally associated 3,461,193 Patented Aug. 12, 1969 with, for example, customary foam extrusion. This characteristic results from the high concentration of liquid necessary to provide sufficiently rapid polymer solidification to freeze into the solidified structure molecular orientaton arising during extrusion and expansion of the polymer. Consequently, high flow rates during extrusion and very narrow extrusion orifices are required in order to maintain an elevated pressure at the entrance to each orifice and the guarantee a precipitous presure-drop across each orifice. Suitable orifices have diameters from about 0.003 to about 0.050 inch (0.076 to 1.270 mm.) and preferably between about 0.005 and 0.030 inch (0.127 to 0.762 mm). In a sheet-forming die the corresponding dimension is the gap-width. As is readily apparent, such small openings can become plugged with only minute solid impurities.

In practical commercial flash-extrusion, an expandable composition is admitted to each extrusion die through point of supply such as a valve. This valve is kept closed until the supply-line or supply-vessel for expandable composition is full and at properly adjusted temperature and pressure. Ordinarily, for reasons which will be hereinafter discussed, there will exist between the point of supply and the point of extrusion of the composition an enclosed path of substantial volume. On opening the valve to begin flash-extrusion, the volume of enclosed path between the valve and the final extrusion orifice must first become filled before steady-state extrusion conditions can be established. This volume is sometimes hereinafter designated the post-valve volume. Because this flashextrusion is characterized by rapid flashing-off of liquid, i.e., by polymer solidification within a small fraction of a second after passage through the extrusion-orifice, the first portion of expandable composition entering the postvalve volume ordinarily forms sufficient solid polymer to plug the extrusion-orifices. This plugging of the orifices is aggravated by the necessity for incorporating within the post-valve volume such devices as filters, pressure-detection probes, flow-distribution plates, and the like. As described in the aforesaid U.S. Patent No. 3,227,794, the post-valve volume preferably includes a pressure-letdown, flow-restricting orifice upstream of the final extrusionorifice when fibrillated plexifilamentary strands are to be produced.

SUMMARY This invention provides a novel procedure for starting the flash-extrusion of expandable compositions without the disruptive formation of solid polymer within the postvalve volume.

According to the procedure of this invention, upon start-up of the process inert gas under pressure is introduced into the enclosed path between the point of supply and the point of extrusion of the expandable composition before the expandable composition is admitted to the enclosed path. Introduction of the gas is continued until the pressure in the enclosed path exceeds the autogenous pressure of the expandable composition but is no greater than the pressure applied to the composition in the supply. Thereafter, the expandable composition is admitted into the enclosed path and is extruded in the customary way. Flashing of the liquid and solidification of the polymer before reaching the point of extrusion are prevented by the cushion of pressurized inert gas in the enclosed path.

DRAWINGS FIGURE 1 is a schematic representation of a flashextrusion system to which the flow-start-up procedure of this invention applies.

FIGURE 2 is a schematic representation of a single extrusion position indicating a supply-valve, an extrusion DESCRIPTION The generalized apparatus to which the procedure of this invention applie is represented by FIGURE 1. A hot, pressurized, liquid-phase expandable composition is contained in a supply-manifold 10. At least one extrusion-position 11 (and ordinarily a plurality of them) is attached to supply-manifold 10. Each extrusion-position 11 has a supply-valve 13 for beginning or terminating the flow of expandable composition through the post-valve volume to one or more extrusion orifices in each extrusion die 15. Pressure on the expandable composition in manifold is at least great enough that, even after the pressure losses resulting from flow through the post-valve volume, the expandable composition is above its autogenous pressure as it enters each exit-orifice in extrusion-dies 15.

As used herein, the term autogenous pressure denotes the minimum pressure on an expandable composition at a given temperature which maintains the liquid state and prevents the formation of an equilibrium vapor-phase.

When a single sharp pressure let-down across only the exit-orifice is all that is required in the flash-extrusion process, it is possible to mount that orifice directly in the wall of manifold 10. Valves 13 are therefore eliminated by substitution of external closure devices, and there is no post-valve volume. This alternative has numerous disadvantages which dictate against it in practical commercial production. First, it is usually necessary to mount external processing apparatus close to the exitface of each die 15, and orifice closure devices would interfere with placement and/or operation of said apparatus. Secondly, without valves 13 at each extrusionposition 11, it is practically impossible to interrupt flow through one extrusion-die 15 (e.g., to service it or clean it) without shutting down the whole system. Supplyvalves 13 also make possible changing the number of operating positions 11 without costly shut-downs. It is also desirable to interpose at least some transfer line between manifold 10 and dies 15 in order that dies 15 can be arranged spatially with respect to one another somewhat independently of manifold limitations. The most compelling reason for creating a post-valve volume is, however, that other functions are usually either desirable or necessary. Thus, it is necessary for product uniformity to measure and control pressure on the expandable composition just before it enters the final extrusion-orifice but after it has experienced all pressures reductions occurring in the post-valve volume. Experience has shown it desirable to provide a filter for the composition just before entering the extrusion-orifices in order to remove any accidental impurities capable of plugging the orifices. Finally, a preferred method of forming fibrillated plexifilamentary strands involves passing the expandable composition through a pressure let-down zone between two orifices in series, which automatically creates at least some post-valve volume. While the existence of a post-valve volume is seen practically to be necessary, it should be kept as small as possible.

FIGURES 2 and 3 are schematic representations, partially in cross-section, of single extrusion-positions 11 adapted to flash-extrusion as defined. Like parts in the figures are designated by the same numerals. Supply-valve 13 is provided for beginning or terminating flow of expandable composition, which ultimately passes through exit-orifices 21 for expansion and solidification of the thermoplastic polymer. The enclosed volume of these systems between supply-valve 13 and exit-orifices 21 is the post-valve volume.

In the extrusion-position 11 of FIGURE 2, adapted to the production of fibrillated plexifilamentary strands, a pressure let-down orifice 23 is provided upstream of exit-orifice 21, a pressure let-down zone 22 being formed therebetween. In continuous operation, pressure on the expandable composition both above and below let-down orifice 23 is above autogenous pressure; but above orifice 23 it is sufiiciently high to maintain a single liquid phase while below it two liquid phases exist comprising very fine droplets of solvent-rich liquid uniformly dispersed in a continuous polymer-rich phase. Regulation of pressure within pressure let-down zone 22 is critical to the provision of suitable fibrillated plexifilamentary strands, and a pressure sensor 24 is provided therein.

The extrusion-position 11 of FIGURE 3, while capable of producing plexifilamentary material, is especially adapted to the production of foamed strands. The only orifices provided are exit-orifices 21, and pressure upstream of these orifices 21 is both superautogenous and high enough to maintain a single liquid phase. Again, pressure upstream of orifices 21 is critical to properties of the foamed strand being produced, and a suitable pressure sensor 24 is provided.

In the apparatus of both FIGURES 2 and 3, a filter 25 is placed upstream of the orifices 21 and 23. Filter 25 is ordinarily a screen-pack with mesh fine enough to prevent the passage of any particles capable of plugging the orifices but with open area very large with respect to orifice-area so as to prevent large relative pressure drops across filters 25. A pressure-gauge 26 may be provided to measure pressure within the post-valve vOlume upstream of the flow-restricting screens and orifices.

Also provided along the post-valve volume can be one or more inlets or outlets for other functions. In the figures, this is represented by a valve-block 31 wherein, for example, valve 28 permits diverting the flow of expandable composition, valve 29 is an inlet for hot solvent used to clean the post-valve volume, and valve 30 admits pressurized inert gas through an injection-probe 32.

Finally, an automatic flow-control valve 27, responsive to a signal generated by pressure-sensor 24 is preferably provided along each extrusion-position 11. It can be above, below, or combined with supply-valve 13. In continuous production, wherein filters 25 may become partially fouled, automatic valve 27 serves to maintain critical pressures at the entrance to exit-orifices 21. When only short production runs designed for testing the effectiveness of start-up procedures are made, or when only a single extrusion-position operates from manifold 10, automatic-valve 27 can be dispensed with so that pressure regulation in manifold 10 is responsive to pressure read by sensor 24.

Although not indicated in the figures, means for heating all surfaces defining the post-valve volume are externally provided to prevent cooling of the expandable composition during operation.

The foregoing has been intended to clarify typical continuous flash-extrusion processes useful in preparing expanded structures of thermoplastic polymer and to describe suitable apparatus at each extrusion-position 11. Before extrusion starts, however, hot pressurized expandable composition in manifold 10 is kept from the postvalve volume by supply-valve 13, the post-valve volume being empty. On opening supply-valve l3, expandable composition rushes into the post-valve volume, solvent flashes off, solid polymer precipitates, and filters 25 and orifices 21 and 23 become fouled and/or plugged.

In the procedure of this invention, inert gas at a pressure usually about the same as that of the expandable composition above supply-valve 13 is fed through a suitable injection probe 32 into the post-valve volume until pressure-gauge 26 registers the required pressure, whereupon supply-valve 13 is opened and nitrogen flow is stopped. The extrusion then proceeds smoothly with the formation of continuous lengths of foams, sponges, or plexifilamentary strands. Use of the inert gas results in successful start-up substantially 100% of the time.

The practice of the invention is uncomplicated and highly advantageous. No orifice-closures are needed. The inert gas, being of low heat capacity, need not be preheated. It is readily available at the required pressures. Substantially none of it dissolves in the expandable composition, and polymer is insoluble in it. Consequently there is no tendency for the gas to dilute the composition which could lead to the formation of fly, i.e. non-continuous webs of moist, tacky, fluffy polymer, upon extrusion.

An inert gas is any material which remains gaseous at the necessary pressures, which is substantially unreactive with both expandable composition and retaining walls, and which is substantially insoluble in the heated composition. Such inert gases are well known. Of them, nitrogen is particularly preferred because it is inexpensive and readily available at the required pressures. Both air and super-heated steam are occasionally suitable inert gases. In some cases, however, oxygen in air can promote excessive corrosion of apparatus; and, in production of plexifilamentary strands, steam interferes with the necessary electrostatic charging of the strands.

Inert gas can be admitted anywhere within the postvalve volume, e.g., through injection probe 32 via valved inlet 30. When the pressure in the post-valve volume (gage 26) exceeds autogenous pressure for the expandable composition, but is no greater than its applied pressure, valve 13 is opened and the flash-extrusion process commences. A preferred probe 32 for injection of inert gas comprises a spring-loaded check valve, e.g., a poppet valve, which is opened by high differential pressure but is automatically closed by the spring when the difference between inlet and outlet pressures becomes less than a predetermined amount, e.g., 100 p.s.i. (7.0 kg./cm. Likewise effective is a three-way valve serving either as inlet 32 for inert gas or as supply-valve 13 depending on which way it is switched. Otherwise, demanding manual manipulation can be required to prevent flow of the composition into the inlet for inert gas.

The inlet 32 for inert gas is preferably upstream of all flow-restricting orifices (e.g., pressure let-down orifice 23) and especially of final filter-screen 25. This location has the advantages that:

(l) accidental solid impurities in the gas stream are removed before reaching orifices 21 or 23; v

(2) the gas expands on contact with the heated filter 25 to generate more back-pressure and reduce the time necessary to reach required pressure in the post-valve volume;

(3) the flow-resistance of filter 25 reduces total gas-flow and subsequent cooling of orifices 21 and 23.

On the other hand, it is frequently desirable to locate inlet 32 for inert gas as far downstream as is consistent with being upstream of the orifices and filter screens, this arrangement permitting gas-flow to continue until expandable composition has a minimum remaining path before reaching exit-orifice 21. The pressure increment for injected inert gas above autogenous pressure for expandable composition should be great enough to maintain at least autogenous pressure on the composition until it reaches exit-orifice 21. This required pressure increment can be determined only by trial for each system. To obviate this determination and also to prevent the possible initial formation of nonstandard product, the maximum pressure exerted by inert gas within the post-valve volume is usually selected to be approximately equal to, but no greater than, the pressure on the supply of expandable composition. Frequently this state is achieved when the static pressure on the supply of inert gas is actually slightly greater than the pressure on the composition.

The start-up procedure of this invention is not intended to be limited only to flash-extrusion as specifically described hereinabove or in the examples. It applies equally to the flash-extrusion of any expandable polymeric composition wherein a volume between supply-valve and exitorifices exists, and in particular to the extrusion of dispersions as well as solutions. The following examples, wherein all parts and percentages are by weight, further illustrate the scope and advantages of this invention.

Example I Fibrillated plexifilamentary strands were to be produced using apparatus substantially as shown in FIG- URE 2. In place of automatic flow-control valve 27, a metering gear-pump was substituted. The uniform solution in the supply-manifold was 12% linear polyethylene and 88% methylene chloride at about 209 C. and under about 860 p.s.i.g. (60.5 kg./cm. gage). On starting the gear-pump and then opening supply-valve 13, the

system immediately plugged. Apparently the methylene.

chloride flashed-off within the pump sufiiciently that the high-viscosity concentrated solution or molten polymer within it caused the pump to stall.

A tank of pressurized nitrogen adjusted to deliver the gas at about 900 p.s.i.g. (63.3 kg./cm. gage) was connected to the system via a needle- valve 30 and 32. Before opening supply-valve 13, nitrogen was admitted to the system until pressure at the gear-pump discharge reached about 900 p.s.i.g. (63 kg./cm. gage), the gearpump motor was turned on, supply-valve 13 was opened, and then quickly and simultaneously the gear-pump was speeded up to deliver solution at about 900 p.s.i.g. (63 kg./cm. gage) and at about 318 grams per 3 minutes, and the nitrogen valve was closed. The extrusion of fibrillated plexifilamentary strand began and continued with no difficulty.

The above comparative results were duplicated in extrusions using solutions of 10 to 12% polymer at supply pressures ranging from 800 to 1000 p.s.i.g. (56 to kg./cm. gage). Pressurizing the die-volume with inert gas prior to opening the supply-valve was shown to effectively and inextensively eliminate plugging associated with the beginning of solution flow.

Example II Apparatus substantially as described in connection with FIGURE 2, but arranged specifically as shown scrematically in FIGURE 4, was provided for the production of fibrillated plexifilamentary strand material. In this arrangement the post-valve volume was that volume of the system between three-way valve 44 and exit-orifice 50. This system differed from that illustrated by FIGURE 2 principally in the use of three-way valve 44. Initially, valve 44 was closed to hot polymer solution in manifold 40 under elevated pressure as indicated by gage 41. Simultaneously, it was open to flow of nitrogen under pressure as indicated by gage 43. Auxiliary valve 42 was provided to begin or terminate nitrogen flow. Also provided, but not shown, was piping for the introduction of hot trichlorofiuoromethane, in place of the nitrogen, for subsequent cleaning of the post-valvevolume. On opening valve 44 to permit passage of polymer solution from manifold 40, valve 44 simultaneously terminated nitrogen flow. Thus, through the preferred use of a three-way valve, valve-block 31 as indicated in FIGURE 2 was eliminated.

The uniform, single-liquid-phase, polymer solution in supply manifold 40 contained 12.8 parts of linear polyethylene per 87.2 parts of trichlorofluoromethane, was at about 185 C., and was under about 1790 p.s.i.g. (126 kg./cm. gage) of pressure. Two filters were employed, the upstream filter 46 using 50 mesh screening and the other filter 47 using -mesh screening (both US. Sieve Series). Let-down orifice 48 was 0.026 inch (0.66 mm.) in diameter and 0.025 inch (0.63 mm.) long. Exit-orifice 50 was 0.020 inch (0.51 mm.) in diameter and 0.025 inch (0.63 mm.) long. A tunnel 51 co-axial with exit-orifice 50 was provided to direct the plexifilamentary strand against an oscillating baflie deflector 53 which in turn directed the strand downward across the surface of charging plate 54 thence onto a moving electrostatically charged collection belt (not shown). The tunnel was 0.125 inch (3.18 mm.) in diameter and 0.1875 inch (4.76 mm.) long.

The purpose of let-down zone 49 was to drop pressure on the single-liquid-phase solution sufficiently that two liquid phases formed. At the solution concentration and temperature specified, pressure required for formation of two liquid phases was below about 1290 p.s.i.g. (90.7 kg./cm. gage) but above the autogenous pressure of approximately 515 p.s.i.g. (36.2 kg./cm. gage). Pressure sensor 52 within let-down zone 49 provided the signal by which automatic flow-control valve 45 maintained a preselected pressure within zone 49.

Solution-flow through the post-valve volume was started as follows:

(1) Automatic flow-control valve 45 was set at its 100% open manual position.

(2) Nitrogen at 1780 p.s.i.g. (125 kg./cm. gage) was admitted via three-way valve 44.

(3) As soon as pressure in let-down zone 49 reached 1500 p.s.i.g. (105 kg./cm. gage), three-way valve 44 was turned to interrupt nitrogen-flow and begin solutionfiow. As solution followed nitrogen through the postvalve volume, momentary pressure fluctuations occurred. Pressure in manifold 40 decreased from 1790 p.s.i.g. (126 kg./cm. gage) to 1730 p.s.i.g. (122 kg./cm. gage), and then returned to 1780 p.s.i.g. (125 kg./cm. gage). Pressure within filter 46 decreased from a maximum of 1745 p.s.i.g. (123 kg./cm. gage) to 1600 p.s.i.g. (112 kg./cm. gage Pressure within let-down zone 49 decreased to a stable value of 1100 p.s.i.g. (77 kg./Crn. gage).

(4) Automatic flow-control valve 45 was adjusted to control the let-down pressure in zone 49 at 950 p.s.i.g. (67 kg./cm. gage).

Solution-flow began and continued with no problems. No fine, tacky, polymer fluff formed; surfaces of oscillating deflector 53 and charging plate 54 remained perfectly clean; and there was no disruption of product laydown on the moving collection belt.

Successive successful start-ups of multiple spaced extrusion positions, as indicated in FIGURE 1, were made using substantially the same conditions and procedures of this example. Wide, uniform, continuous, plexifilamentary, nonwoven polyethylene sheets were thereby produced.

Example III Apparatus similar to that represented by FIGURE 3 was attached to a supply of expandable composition at conditions selected to produce an ultramicrocellular foam.

The expandable compositon was a solution of polyethylene terephthalate in methylene chloride at a 60/40 (polymer/ solvent) ratio. Relative viscosity of the polymer was 54.8 (relative viscosity is here defined as the ratio of absolute viscosities at 25 C. of an 8.7% polymer solution and its solvent, the solvent being composed of 70 parts of 2,4,6-trichlorophenol in 100'parts of phenol). The uniform solution continuously discharged from a mixer was collected in a variable-volume cylindrical accumulator closed at its top with a hydraulically operated piston. The apparatus was attached at the bottom of the accumulator.

Only one extrusion position 11 with a single orifice 21 was employed. Automatic-valve 27 was thus unnecessary and was not provided. Nor was a valve-block 31, as shown, provided. The die-assembly containing screenpack 25 and an orifice-plate was fastened directly to the downstream leg of ball-valve (supply-valve) 13, and suitable pressure sensing devices were provided along the length of the extrusion-position 11. The single orifice 21 was 0.012 inch (0.305 mm.) in diameter with a length-todiameter (L/D) ratio of- 1.0. The screen-pack was a 200- mesh screen covered with a 20-mesh screen and supported from below with two layers of 20-mesh screen (all U.S. Sieve Series). A thermocouple for measuring solution temperature was mounted in the extrusion die.

Pressure on the-solution in the accumulator was 1600 p.s.i.g. (112.5 kg./cm. gage). A nitrogen supply was adjusted to supply nitrogen at about 1500 p.s.i.g. (105.5 kg./cm. gage). Nitrogen flow was begun through on opening upstream of screen-pack 25 to pressurize the system below supply-valve 13.

When nitrogen-flow had been established and the supply valve was opened, extrusion of the polymer-solution started perfectly and continued so until the end of the supply of solution. As soon as flow was established the pressure applied to solution in the accumulator was reduced to about 850 p.s.i.g. (59.8 kg./cm. gage) to produce asolution pressure just upstream of the orifice of about 800 p.s.i.g. (56.2 kg./cm. gage). The solution temperature was about 214 C. The product was a continuous filament of ultramicrocellular polyethylene terepthalate.

Example IV Another extrusion start-up was attempted using substantialy the same equipment, procedure and conditions as in Example III. The orifice-plate in this case, however, had four spaced orifices 0.012 inch (0.305 mm.) in diameter (L/D=1.0). Upstream of the orifice-plate but downstream of the screen-pack was interposed a metering plate with four holes axially aligned with those in the orificeplate. Each of these holes was 0.025 inch (0.635 mm.) in diameter with L/ D=5 .0. The expandable solution was polyethylene terephthalate and 35% methylene chloride. A valved nitrogen supply at 1800 p.s.i.g. (126.5 kg./cm. gage) was connected via a pressure-relief valve to the opening just downstream of the supply-valve but well upstream of the screen-pack. Pressure on the solution in the accumulator was also about 1800 p.s.i.g. (126.5 kg./cm. gage), but a pressure-drop of about p.s.i.g. (7 kg/cm? gage) on nitrogen-flow through the pressure-relief valve was known to exist, thus guaranteeing that nitrogen pressure downstream of the supply-valve would not exceed the upstream solution pressure. Solutign temperature above the supply-valve was about 215 2 0 C.

In quick succession, nitrogen flow was begun until pressure inside the flow system reached a maximum, the supply-valve was opened, and the nitrogen-valve was closed. Nitrogen issued for a few seconds from the orifices, followed immediately by four foamed filaments. After about 10 minutes operation, three samples of prod-, uct issuing from each orifice were collected. Each was rendered fully inflated by brief immersion in liquid 1,1,2- trichloro-l,2,2-trifluoroethane followed by drying in ambient air. Measured diameters for these samples were:

Average diameter Individual diameters Orifice N0. in. mm (in.)

I 054 l. 37 .055, .054, .053 2 055 l. 41 .056, .055, .055 3 055 1. 40 .056, .054, .055 4 053 1.35 .053, .053, .053

The above results show excellent orifice-to-orifice uniformity and completely successful start-up. Solution temperature near the orifice-plate was about 215 C.

Example V I 9 I claim: 1. In a flash extrusion process wherein an expandable composition comprising a thermoplastic polymer and a liquid at a temperature above that at which the polymer freezes out and under a pressure above the autogenous pressure of the composition is fed from a supply and extruded into a region of reduced pressure causing the liquid to flash and the polymer to solidify, and wherein an enclosed path of substantial volume exists between the point of supply and the point of extrusion, the improvement which comprises, upon start-up of the process, introducing an inert gas into the enclosed path.

prior to admitting the expandable composition to the enclosed path until the pressure of the gas in the enclosed path is in excess of the autogenous pressure of the composition but no greater than the pressure applied to the composition in the supply UNiTED STATES PATENTS 2,330,932 10/1943 Taylor et a1. 264-85 XR 3,102,365 9/1963 Sneary et al 264-53 XR 3,215,760 11/1965 Grace et a1 26485 XR 3,227,784 1/1966 Blades et al. 264-53 3,389,198 6/1968 Taber 26488 XR PHILIP E. ANDERSON, Primary Examiner US, Cl. X.R.

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